/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
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/*
 *
 *
 *
 *
 *
 * Written by Doug Lea and Martin Buchholz with assistance from members of
 * JCP JSR-166 Expert Group and released to the public domain, as explained
 * at http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;

import java.util.AbstractCollection;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Deque;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Queue;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.function.Consumer;

/**
 * An unbounded concurrent {@linkplain Deque deque} based on linked nodes.
 * Concurrent insertion, removal, and access operations execute safely
 * across multiple threads.
 * A {@code ConcurrentLinkedDeque} is an appropriate choice when
 * many threads will share access to a common collection.
 * Like most other concurrent collection implementations, this class
 * does not permit the use of {@code null} elements.
 *
 * <p>Iterators and spliterators are
 * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
 *
 * <p>Beware that, unlike in most collections, the {@code size} method
 * is <em>NOT</em> a constant-time operation. Because of the
 * asynchronous nature of these deques, determining the current number
 * of elements requires a traversal of the elements, and so may report
 * inaccurate results if this collection is modified during traversal.
 * Additionally, the bulk operations {@code addAll},
 * {@code removeAll}, {@code retainAll}, {@code containsAll},
 * {@code equals}, and {@code toArray} are <em>not</em> guaranteed
 * to be performed atomically. For example, an iterator operating
 * concurrently with an {@code addAll} operation might view only some
 * of the added elements.
 *
 * <p>This class and its iterator implement all of the <em>optional</em>
 * methods of the {@link Deque} and {@link Iterator} interfaces.
 *
 * <p>Memory consistency effects: As with other concurrent collections,
 * actions in a thread prior to placing an object into a
 * {@code ConcurrentLinkedDeque}
 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * actions subsequent to the access or removal of that element from
 * the {@code ConcurrentLinkedDeque} in another thread.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @param <E> the type of elements held in this collection
 * @author Doug Lea
 * @author Martin Buchholz
 * @since 1.7
 */
public class ConcurrentLinkedDeque<E>
    extends AbstractCollection<E>
    implements Deque<E>, java.io.Serializable {

    /*
     * This is an implementation of a concurrent lock-free deque
     * supporting interior removes but not interior insertions, as
     * required to support the entire Deque interface.
     *
     * We extend the techniques developed for ConcurrentLinkedQueue and
     * LinkedTransferQueue (see the internal docs for those classes).
     * Understanding the ConcurrentLinkedQueue implementation is a
     * prerequisite for understanding the implementation of this class.
     *
     * The data structure is a symmetrical doubly-linked "GC-robust"
     * linked list of nodes.  We minimize the number of volatile writes
     * using two techniques: advancing multiple hops with a single CAS
     * and mixing volatile and non-volatile writes of the same memory
     * locations.
     *
     * A node contains the expected E ("item") and links to predecessor
     * ("prev") and successor ("next") nodes:
     *
     * class Node<E> { volatile Node<E> prev, next; volatile E item; }
     *
     * A node p is considered "live" if it contains a non-null item
     * (p.item != null).  When an item is CASed to null, the item is
     * atomically logically deleted from the collection.
     *
     * At any time, there is precisely one "first" node with a null
     * prev reference that terminates any chain of prev references
     * starting at a live node.  Similarly there is precisely one
     * "last" node terminating any chain of next references starting at
     * a live node.  The "first" and "last" nodes may or may not be live.
     * The "first" and "last" nodes are always mutually reachable.
     *
     * A new element is added atomically by CASing the null prev or
     * next reference in the first or last node to a fresh node
     * containing the element.  The element's node atomically becomes
     * "live" at that point.
     *
     * A node is considered "active" if it is a live node, or the
     * first or last node.  Active nodes cannot be unlinked.
     *
     * A "self-link" is a next or prev reference that is the same node:
     *   p.prev == p  or  p.next == p
     * Self-links are used in the node unlinking process.  Active nodes
     * never have self-links.
     *
     * A node p is active if and only if:
     *
     * p.item != null ||
     * (p.prev == null && p.next != p) ||
     * (p.next == null && p.prev != p)
     *
     * The deque object has two node references, "head" and "tail".
     * The head and tail are only approximations to the first and last
     * nodes of the deque.  The first node can always be found by
     * following prev pointers from head; likewise for tail.  However,
     * it is permissible for head and tail to be referring to deleted
     * nodes that have been unlinked and so may not be reachable from
     * any live node.
     *
     * There are 3 stages of node deletion;
     * "logical deletion", "unlinking", and "gc-unlinking".
     *
     * 1. "logical deletion" by CASing item to null atomically removes
     * the element from the collection, and makes the containing node
     * eligible for unlinking.
     *
     * 2. "unlinking" makes a deleted node unreachable from active
     * nodes, and thus eventually reclaimable by GC.  Unlinked nodes
     * may remain reachable indefinitely from an iterator.
     *
     * Physical node unlinking is merely an optimization (albeit a
     * critical one), and so can be performed at our convenience.  At
     * any time, the set of live nodes maintained by prev and next
     * links are identical, that is, the live nodes found via next
     * links from the first node is equal to the elements found via
     * prev links from the last node.  However, this is not true for
     * nodes that have already been logically deleted - such nodes may
     * be reachable in one direction only.
     *
     * 3. "gc-unlinking" takes unlinking further by making active
     * nodes unreachable from deleted nodes, making it easier for the
     * GC to reclaim future deleted nodes.  This step makes the data
     * structure "gc-robust", as first described in detail by Boehm
     * (http://portal.acm.org/citation.cfm?doid=503272.503282).
     *
     * GC-unlinked nodes may remain reachable indefinitely from an
     * iterator, but unlike unlinked nodes, are never reachable from
     * head or tail.
     *
     * Making the data structure GC-robust will eliminate the risk of
     * unbounded memory retention with conservative GCs and is likely
     * to improve performance with generational GCs.
     *
     * When a node is dequeued at either end, e.g. via poll(), we would
     * like to break any references from the node to active nodes.  We
     * develop further the use of self-links that was very effective in
     * other concurrent collection classes.  The idea is to replace
     * prev and next pointers with special values that are interpreted
     * to mean off-the-list-at-one-end.  These are approximations, but
     * good enough to preserve the properties we want in our
     * traversals, e.g. we guarantee that a traversal will never visit
     * the same element twice, but we don't guarantee whether a
     * traversal that runs out of elements will be able to see more
     * elements later after enqueues at that end.  Doing gc-unlinking
     * safely is particularly tricky, since any node can be in use
     * indefinitely (for example by an iterator).  We must ensure that
     * the nodes pointed at by head/tail never get gc-unlinked, since
     * head/tail are needed to get "back on track" by other nodes that
     * are gc-unlinked.  gc-unlinking accounts for much of the
     * implementation complexity.
     *
     * Since neither unlinking nor gc-unlinking are necessary for
     * correctness, there are many implementation choices regarding
     * frequency (eagerness) of these operations.  Since volatile
     * reads are likely to be much cheaper than CASes, saving CASes by
     * unlinking multiple adjacent nodes at a time may be a win.
     * gc-unlinking can be performed rarely and still be effective,
     * since it is most important that long chains of deleted nodes
     * are occasionally broken.
     *
     * The actual representation we use is that p.next == p means to
     * goto the first node (which in turn is reached by following prev
     * pointers from head), and p.next == null && p.prev == p means
     * that the iteration is at an end and that p is a (static final)
     * dummy node, NEXT_TERMINATOR, and not the last active node.
     * Finishing the iteration when encountering such a TERMINATOR is
     * good enough for read-only traversals, so such traversals can use
     * p.next == null as the termination condition.  When we need to
     * find the last (active) node, for enqueueing a new node, we need
     * to check whether we have reached a TERMINATOR node; if so,
     * restart traversal from tail.
     *
     * The implementation is completely directionally symmetrical,
     * except that most public methods that iterate through the list
     * follow next pointers ("forward" direction).
     *
     * We believe (without full proof) that all single-element deque
     * operations (e.g., addFirst, peekLast, pollLast) are linearizable
     * (see Herlihy and Shavit's book).  However, some combinations of
     * operations are known not to be linearizable.  In particular,
     * when an addFirst(A) is racing with pollFirst() removing B, it is
     * possible for an observer iterating over the elements to observe
     * A B C and subsequently observe A C, even though no interior
     * removes are ever performed.  Nevertheless, iterators behave
     * reasonably, providing the "weakly consistent" guarantees.
     *
     * Empirically, microbenchmarks suggest that this class adds about
     * 40% overhead relative to ConcurrentLinkedQueue, which feels as
     * good as we can hope for.
     */

  private static final long serialVersionUID = 876323262645176354L;

  /**
   * A node from which the first node on list (that is, the unique node p
   * with p.prev == null && p.next != p) can be reached in O(1) time.
   * Invariants:
   * - the first node is always O(1) reachable from head via prev links
   * - all live nodes are reachable from the first node via succ()
   * - head != null
   * - (tmp = head).next != tmp || tmp != head
   * - head is never gc-unlinked (but may be unlinked)
   * Non-invariants:
   * - head.item may or may not be null
   * - head may not be reachable from the first or last node, or from tail
   */
  private transient volatile Node<E> head;

  /**
   * A node from which the last node on list (that is, the unique node p
   * with p.next == null && p.prev != p) can be reached in O(1) time.
   * Invariants:
   * - the last node is always O(1) reachable from tail via next links
   * - all live nodes are reachable from the last node via pred()
   * - tail != null
   * - tail is never gc-unlinked (but may be unlinked)
   * Non-invariants:
   * - tail.item may or may not be null
   * - tail may not be reachable from the first or last node, or from head
   */
  private transient volatile Node<E> tail;

  private static final Node<Object> PREV_TERMINATOR, NEXT_TERMINATOR;

  @SuppressWarnings("unchecked")
  Node<E> prevTerminator() {
    return (Node<E>) PREV_TERMINATOR;
  }

  @SuppressWarnings("unchecked")
  Node<E> nextTerminator() {
    return (Node<E>) NEXT_TERMINATOR;
  }

  static final class Node<E> {

    volatile Node<E> prev;
    volatile E item;
    volatile Node<E> next;

    Node() {  // default constructor for NEXT_TERMINATOR, PREV_TERMINATOR
    }

    /**
     * Constructs a new node.  Uses relaxed write because item can
     * only be seen after publication via casNext or casPrev.
     */
    Node(E item) {
      UNSAFE.putObject(this, itemOffset, item);
    }

    boolean casItem(E cmp, E val) {
      return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
    }

    void lazySetNext(Node<E> val) {
      UNSAFE.putOrderedObject(this, nextOffset, val);
    }

    boolean casNext(Node<E> cmp, Node<E> val) {
      return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
    }

    void lazySetPrev(Node<E> val) {
      UNSAFE.putOrderedObject(this, prevOffset, val);
    }

    boolean casPrev(Node<E> cmp, Node<E> val) {
      return UNSAFE.compareAndSwapObject(this, prevOffset, cmp, val);
    }

    // Unsafe mechanics

    private static final sun.misc.Unsafe UNSAFE;
    private static final long prevOffset;
    private static final long itemOffset;
    private static final long nextOffset;

    static {
      try {
        UNSAFE = sun.misc.Unsafe.getUnsafe();
        Class<?> k = Node.class;
        prevOffset = UNSAFE.objectFieldOffset
            (k.getDeclaredField("prev"));
        itemOffset = UNSAFE.objectFieldOffset
            (k.getDeclaredField("item"));
        nextOffset = UNSAFE.objectFieldOffset
            (k.getDeclaredField("next"));
      } catch (Exception e) {
        throw new Error(e);
      }
    }
  }

  /**
   * Links e as first element.
   */
  private void linkFirst(E e) {
    checkNotNull(e);
    final Node<E> newNode = new Node<E>(e);

    restartFromHead:
    for (; ; ) {
      for (Node<E> h = head, p = h, q; ; ) {
        if ((q = p.prev) != null &&
            (q = (p = q).prev) != null)
        // Check for head updates every other hop.
        // If p == q, we are sure to follow head instead.
        {
          p = (h != (h = head)) ? h : q;
        } else if (p.next == p) // PREV_TERMINATOR
        {
          continue restartFromHead;
        } else {
          // p is first node
          newNode.lazySetNext(p); // CAS piggyback
          if (p.casPrev(null, newNode)) {
            // Successful CAS is the linearization point
            // for e to become an element of this deque,
            // and for newNode to become "live".
            if (p != h) // hop two nodes at a time
            {
              casHead(h, newNode);  // Failure is OK.
            }
            return;
          }
          // Lost CAS race to another thread; re-read prev
        }
      }
    }
  }

  /**
   * Links e as last element.
   */
  private void linkLast(E e) {
    checkNotNull(e);
    final Node<E> newNode = new Node<E>(e);

    restartFromTail:
    for (; ; ) {
      for (Node<E> t = tail, p = t, q; ; ) {
        if ((q = p.next) != null &&
            (q = (p = q).next) != null)
        // Check for tail updates every other hop.
        // If p == q, we are sure to follow tail instead.
        {
          p = (t != (t = tail)) ? t : q;
        } else if (p.prev == p) // NEXT_TERMINATOR
        {
          continue restartFromTail;
        } else {
          // p is last node
          newNode.lazySetPrev(p); // CAS piggyback
          if (p.casNext(null, newNode)) {
            // Successful CAS is the linearization point
            // for e to become an element of this deque,
            // and for newNode to become "live".
            if (p != t) // hop two nodes at a time
            {
              casTail(t, newNode);  // Failure is OK.
            }
            return;
          }
          // Lost CAS race to another thread; re-read next
        }
      }
    }
  }

  private static final int HOPS = 2;

  /**
   * Unlinks non-null node x.
   */
  void unlink(Node<E> x) {
    // assert x != null;
    // assert x.item == null;
    // assert x != PREV_TERMINATOR;
    // assert x != NEXT_TERMINATOR;

    final Node<E> prev = x.prev;
    final Node<E> next = x.next;
    if (prev == null) {
      unlinkFirst(x, next);
    } else if (next == null) {
      unlinkLast(x, prev);
    } else {
      // Unlink interior node.
      //
      // This is the common case, since a series of polls at the
      // same end will be "interior" removes, except perhaps for
      // the first one, since end nodes cannot be unlinked.
      //
      // At any time, all active nodes are mutually reachable by
      // following a sequence of either next or prev pointers.
      //
      // Our strategy is to find the unique active predecessor
      // and successor of x.  Try to fix up their links so that
      // they point to each other, leaving x unreachable from
      // active nodes.  If successful, and if x has no live
      // predecessor/successor, we additionally try to gc-unlink,
      // leaving active nodes unreachable from x, by rechecking
      // that the status of predecessor and successor are
      // unchanged and ensuring that x is not reachable from
      // tail/head, before setting x's prev/next links to their
      // logical approximate replacements, self/TERMINATOR.
      Node<E> activePred, activeSucc;
      boolean isFirst, isLast;
      int hops = 1;

      // Find active predecessor
      for (Node<E> p = prev; ; ++hops) {
        if (p.item != null) {
          activePred = p;
          isFirst = false;
          break;
        }
        Node<E> q = p.prev;
        if (q == null) {
          if (p.next == p) {
            return;
          }
          activePred = p;
          isFirst = true;
          break;
        } else if (p == q) {
          return;
        } else {
          p = q;
        }
      }

      // Find active successor
      for (Node<E> p = next; ; ++hops) {
        if (p.item != null) {
          activeSucc = p;
          isLast = false;
          break;
        }
        Node<E> q = p.next;
        if (q == null) {
          if (p.prev == p) {
            return;
          }
          activeSucc = p;
          isLast = true;
          break;
        } else if (p == q) {
          return;
        } else {
          p = q;
        }
      }

      // TODO: better HOP heuristics
      if (hops < HOPS
          // always squeeze out interior deleted nodes
          && (isFirst | isLast)) {
        return;
      }

      // Squeeze out deleted nodes between activePred and
      // activeSucc, including x.
      skipDeletedSuccessors(activePred);
      skipDeletedPredecessors(activeSucc);

      // Try to gc-unlink, if possible
      if ((isFirst | isLast) &&

          // Recheck expected state of predecessor and successor
          (activePred.next == activeSucc) &&
          (activeSucc.prev == activePred) &&
          (isFirst ? activePred.prev == null : activePred.item != null) &&
          (isLast ? activeSucc.next == null : activeSucc.item != null)) {

        updateHead(); // Ensure x is not reachable from head
        updateTail(); // Ensure x is not reachable from tail

        // Finally, actually gc-unlink
        x.lazySetPrev(isFirst ? prevTerminator() : x);
        x.lazySetNext(isLast ? nextTerminator() : x);
      }
    }
  }

  /**
   * Unlinks non-null first node.
   */
  private void unlinkFirst(Node<E> first, Node<E> next) {
    // assert first != null;
    // assert next != null;
    // assert first.item == null;
    for (Node<E> o = null, p = next, q; ; ) {
      if (p.item != null || (q = p.next) == null) {
        if (o != null && p.prev != p && first.casNext(next, p)) {
          skipDeletedPredecessors(p);
          if (first.prev == null &&
              (p.next == null || p.item != null) &&
              p.prev == first) {

            updateHead(); // Ensure o is not reachable from head
            updateTail(); // Ensure o is not reachable from tail

            // Finally, actually gc-unlink
            o.lazySetNext(o);
            o.lazySetPrev(prevTerminator());
          }
        }
        return;
      } else if (p == q) {
        return;
      } else {
        o = p;
        p = q;
      }
    }
  }

  /**
   * Unlinks non-null last node.
   */
  private void unlinkLast(Node<E> last, Node<E> prev) {
    // assert last != null;
    // assert prev != null;
    // assert last.item == null;
    for (Node<E> o = null, p = prev, q; ; ) {
      if (p.item != null || (q = p.prev) == null) {
        if (o != null && p.next != p && last.casPrev(prev, p)) {
          skipDeletedSuccessors(p);
          if (last.next == null &&
              (p.prev == null || p.item != null) &&
              p.next == last) {

            updateHead(); // Ensure o is not reachable from head
            updateTail(); // Ensure o is not reachable from tail

            // Finally, actually gc-unlink
            o.lazySetPrev(o);
            o.lazySetNext(nextTerminator());
          }
        }
        return;
      } else if (p == q) {
        return;
      } else {
        o = p;
        p = q;
      }
    }
  }

  /**
   * Guarantees that any node which was unlinked before a call to
   * this method will be unreachable from head after it returns.
   * Does not guarantee to eliminate slack, only that head will
   * point to a node that was active while this method was running.
   */
  private final void updateHead() {
    // Either head already points to an active node, or we keep
    // trying to cas it to the first node until it does.
    Node<E> h, p, q;
    restartFromHead:
    while ((h = head).item == null && (p = h.prev) != null) {
      for (; ; ) {
        if ((q = p.prev) == null ||
            (q = (p = q).prev) == null) {
          // It is possible that p is PREV_TERMINATOR,
          // but if so, the CAS is guaranteed to fail.
          if (casHead(h, p)) {
            return;
          } else {
            continue restartFromHead;
          }
        } else if (h != head) {
          continue restartFromHead;
        } else {
          p = q;
        }
      }
    }
  }

  /**
   * Guarantees that any node which was unlinked before a call to
   * this method will be unreachable from tail after it returns.
   * Does not guarantee to eliminate slack, only that tail will
   * point to a node that was active while this method was running.
   */
  private final void updateTail() {
    // Either tail already points to an active node, or we keep
    // trying to cas it to the last node until it does.
    Node<E> t, p, q;
    restartFromTail:
    while ((t = tail).item == null && (p = t.next) != null) {
      for (; ; ) {
        if ((q = p.next) == null ||
            (q = (p = q).next) == null) {
          // It is possible that p is NEXT_TERMINATOR,
          // but if so, the CAS is guaranteed to fail.
          if (casTail(t, p)) {
            return;
          } else {
            continue restartFromTail;
          }
        } else if (t != tail) {
          continue restartFromTail;
        } else {
          p = q;
        }
      }
    }
  }

  private void skipDeletedPredecessors(Node<E> x) {
    whileActive:
    do {
      Node<E> prev = x.prev;
      // assert prev != null;
      // assert x != NEXT_TERMINATOR;
      // assert x != PREV_TERMINATOR;
      Node<E> p = prev;
      findActive:
      for (; ; ) {
        if (p.item != null) {
          break findActive;
        }
        Node<E> q = p.prev;
        if (q == null) {
          if (p.next == p) {
            continue whileActive;
          }
          break findActive;
        } else if (p == q) {
          continue whileActive;
        } else {
          p = q;
        }
      }

      // found active CAS target
      if (prev == p || x.casPrev(prev, p)) {
        return;
      }

    } while (x.item != null || x.next == null);
  }

  private void skipDeletedSuccessors(Node<E> x) {
    whileActive:
    do {
      Node<E> next = x.next;
      // assert next != null;
      // assert x != NEXT_TERMINATOR;
      // assert x != PREV_TERMINATOR;
      Node<E> p = next;
      findActive:
      for (; ; ) {
        if (p.item != null) {
          break findActive;
        }
        Node<E> q = p.next;
        if (q == null) {
          if (p.prev == p) {
            continue whileActive;
          }
          break findActive;
        } else if (p == q) {
          continue whileActive;
        } else {
          p = q;
        }
      }

      // found active CAS target
      if (next == p || x.casNext(next, p)) {
        return;
      }

    } while (x.item != null || x.prev == null);
  }

  /**
   * Returns the successor of p, or the first node if p.next has been
   * linked to self, which will only be true if traversing with a
   * stale pointer that is now off the list.
   */
  final Node<E> succ(Node<E> p) {
    // TODO: should we skip deleted nodes here?
    Node<E> q = p.next;
    return (p == q) ? first() : q;
  }

  /**
   * Returns the predecessor of p, or the last node if p.prev has been
   * linked to self, which will only be true if traversing with a
   * stale pointer that is now off the list.
   */
  final Node<E> pred(Node<E> p) {
    Node<E> q = p.prev;
    return (p == q) ? last() : q;
  }

  /**
   * Returns the first node, the unique node p for which:
   * p.prev == null && p.next != p
   * The returned node may or may not be logically deleted.
   * Guarantees that head is set to the returned node.
   */
  Node<E> first() {
    restartFromHead:
    for (; ; ) {
      for (Node<E> h = head, p = h, q; ; ) {
        if ((q = p.prev) != null &&
            (q = (p = q).prev) != null)
        // Check for head updates every other hop.
        // If p == q, we are sure to follow head instead.
        {
          p = (h != (h = head)) ? h : q;
        } else if (p == h
            // It is possible that p is PREV_TERMINATOR,
            // but if so, the CAS is guaranteed to fail.
            || casHead(h, p)) {
          return p;
        } else {
          continue restartFromHead;
        }
      }
    }
  }

  /**
   * Returns the last node, the unique node p for which:
   * p.next == null && p.prev != p
   * The returned node may or may not be logically deleted.
   * Guarantees that tail is set to the returned node.
   */
  Node<E> last() {
    restartFromTail:
    for (; ; ) {
      for (Node<E> t = tail, p = t, q; ; ) {
        if ((q = p.next) != null &&
            (q = (p = q).next) != null)
        // Check for tail updates every other hop.
        // If p == q, we are sure to follow tail instead.
        {
          p = (t != (t = tail)) ? t : q;
        } else if (p == t
            // It is possible that p is NEXT_TERMINATOR,
            // but if so, the CAS is guaranteed to fail.
            || casTail(t, p)) {
          return p;
        } else {
          continue restartFromTail;
        }
      }
    }
  }

  // Minor convenience utilities

  /**
   * Throws NullPointerException if argument is null.
   *
   * @param v the element
   */
  private static void checkNotNull(Object v) {
    if (v == null) {
      throw new NullPointerException();
    }
  }

  /**
   * Returns element unless it is null, in which case throws
   * NoSuchElementException.
   *
   * @param v the element
   * @return the element
   */
  private E screenNullResult(E v) {
    if (v == null) {
      throw new NoSuchElementException();
    }
    return v;
  }

  /**
   * Creates an array list and fills it with elements of this list.
   * Used by toArray.
   *
   * @return the array list
   */
  private ArrayList<E> toArrayList() {
    ArrayList<E> list = new ArrayList<E>();
    for (Node<E> p = first(); p != null; p = succ(p)) {
      E item = p.item;
      if (item != null) {
        list.add(item);
      }
    }
    return list;
  }

  /**
   * Constructs an empty deque.
   */
  public ConcurrentLinkedDeque() {
    head = tail = new Node<E>(null);
  }

  /**
   * Constructs a deque initially containing the elements of
   * the given collection, added in traversal order of the
   * collection's iterator.
   *
   * @param c the collection of elements to initially contain
   * @throws NullPointerException if the specified collection or any of its elements are null
   */
  public ConcurrentLinkedDeque(Collection<? extends E> c) {
    // Copy c into a private chain of Nodes
    Node<E> h = null, t = null;
    for (E e : c) {
      checkNotNull(e);
      Node<E> newNode = new Node<E>(e);
      if (h == null) {
        h = t = newNode;
      } else {
        t.lazySetNext(newNode);
        newNode.lazySetPrev(t);
        t = newNode;
      }
    }
    initHeadTail(h, t);
  }

  /**
   * Initializes head and tail, ensuring invariants hold.
   */
  private void initHeadTail(Node<E> h, Node<E> t) {
    if (h == t) {
      if (h == null) {
        h = t = new Node<E>(null);
      } else {
        // Avoid edge case of a single Node with non-null item.
        Node<E> newNode = new Node<E>(null);
        t.lazySetNext(newNode);
        newNode.lazySetPrev(t);
        t = newNode;
      }
    }
    head = h;
    tail = t;
  }

  /**
   * Inserts the specified element at the front of this deque.
   * As the deque is unbounded, this method will never throw
   * {@link IllegalStateException}.
   *
   * @throws NullPointerException if the specified element is null
   */
  public void addFirst(E e) {
    linkFirst(e);
  }

  /**
   * Inserts the specified element at the end of this deque.
   * As the deque is unbounded, this method will never throw
   * {@link IllegalStateException}.
   *
   * <p>This method is equivalent to {@link #add}.
   *
   * @throws NullPointerException if the specified element is null
   */
  public void addLast(E e) {
    linkLast(e);
  }

  /**
   * Inserts the specified element at the front of this deque.
   * As the deque is unbounded, this method will never return {@code false}.
   *
   * @return {@code true} (as specified by {@link Deque#offerFirst})
   * @throws NullPointerException if the specified element is null
   */
  public boolean offerFirst(E e) {
    linkFirst(e);
    return true;
  }

  /**
   * Inserts the specified element at the end of this deque.
   * As the deque is unbounded, this method will never return {@code false}.
   *
   * <p>This method is equivalent to {@link #add}.
   *
   * @return {@code true} (as specified by {@link Deque#offerLast})
   * @throws NullPointerException if the specified element is null
   */
  public boolean offerLast(E e) {
    linkLast(e);
    return true;
  }

  public E peekFirst() {
    for (Node<E> p = first(); p != null; p = succ(p)) {
      E item = p.item;
      if (item != null) {
        return item;
      }
    }
    return null;
  }

  public E peekLast() {
    for (Node<E> p = last(); p != null; p = pred(p)) {
      E item = p.item;
      if (item != null) {
        return item;
      }
    }
    return null;
  }

  /**
   * @throws NoSuchElementException {@inheritDoc}
   */
  public E getFirst() {
    return screenNullResult(peekFirst());
  }

  /**
   * @throws NoSuchElementException {@inheritDoc}
   */
  public E getLast() {
    return screenNullResult(peekLast());
  }

  public E pollFirst() {
    for (Node<E> p = first(); p != null; p = succ(p)) {
      E item = p.item;
      if (item != null && p.casItem(item, null)) {
        unlink(p);
        return item;
      }
    }
    return null;
  }

  public E pollLast() {
    for (Node<E> p = last(); p != null; p = pred(p)) {
      E item = p.item;
      if (item != null && p.casItem(item, null)) {
        unlink(p);
        return item;
      }
    }
    return null;
  }

  /**
   * @throws NoSuchElementException {@inheritDoc}
   */
  public E removeFirst() {
    return screenNullResult(pollFirst());
  }

  /**
   * @throws NoSuchElementException {@inheritDoc}
   */
  public E removeLast() {
    return screenNullResult(pollLast());
  }

  // *** Queue and stack methods ***

  /**
   * Inserts the specified element at the tail of this deque.
   * As the deque is unbounded, this method will never return {@code false}.
   *
   * @return {@code true} (as specified by {@link Queue#offer})
   * @throws NullPointerException if the specified element is null
   */
  public boolean offer(E e) {
    return offerLast(e);
  }

  /**
   * Inserts the specified element at the tail of this deque.
   * As the deque is unbounded, this method will never throw
   * {@link IllegalStateException} or return {@code false}.
   *
   * @return {@code true} (as specified by {@link Collection#add})
   * @throws NullPointerException if the specified element is null
   */
  public boolean add(E e) {
    return offerLast(e);
  }

  public E poll() {
    return pollFirst();
  }

  public E peek() {
    return peekFirst();
  }

  /**
   * @throws NoSuchElementException {@inheritDoc}
   */
  public E remove() {
    return removeFirst();
  }

  /**
   * @throws NoSuchElementException {@inheritDoc}
   */
  public E pop() {
    return removeFirst();
  }

  /**
   * @throws NoSuchElementException {@inheritDoc}
   */
  public E element() {
    return getFirst();
  }

  /**
   * @throws NullPointerException {@inheritDoc}
   */
  public void push(E e) {
    addFirst(e);
  }

  /**
   * Removes the first element {@code e} such that
   * {@code o.equals(e)}, if such an element exists in this deque.
   * If the deque does not contain the element, it is unchanged.
   *
   * @param o element to be removed from this deque, if present
   * @return {@code true} if the deque contained the specified element
   * @throws NullPointerException if the specified element is null
   */
  public boolean removeFirstOccurrence(Object o) {
    checkNotNull(o);
    for (Node<E> p = first(); p != null; p = succ(p)) {
      E item = p.item;
      if (item != null && o.equals(item) && p.casItem(item, null)) {
        unlink(p);
        return true;
      }
    }
    return false;
  }

  /**
   * Removes the last element {@code e} such that
   * {@code o.equals(e)}, if such an element exists in this deque.
   * If the deque does not contain the element, it is unchanged.
   *
   * @param o element to be removed from this deque, if present
   * @return {@code true} if the deque contained the specified element
   * @throws NullPointerException if the specified element is null
   */
  public boolean removeLastOccurrence(Object o) {
    checkNotNull(o);
    for (Node<E> p = last(); p != null; p = pred(p)) {
      E item = p.item;
      if (item != null && o.equals(item) && p.casItem(item, null)) {
        unlink(p);
        return true;
      }
    }
    return false;
  }

  /**
   * Returns {@code true} if this deque contains at least one
   * element {@code e} such that {@code o.equals(e)}.
   *
   * @param o element whose presence in this deque is to be tested
   * @return {@code true} if this deque contains the specified element
   */
  public boolean contains(Object o) {
    if (o == null) {
      return false;
    }
    for (Node<E> p = first(); p != null; p = succ(p)) {
      E item = p.item;
      if (item != null && o.equals(item)) {
        return true;
      }
    }
    return false;
  }

  /**
   * Returns {@code true} if this collection contains no elements.
   *
   * @return {@code true} if this collection contains no elements
   */
  public boolean isEmpty() {
    return peekFirst() == null;
  }

  /**
   * Returns the number of elements in this deque.  If this deque
   * contains more than {@code Integer.MAX_VALUE} elements, it
   * returns {@code Integer.MAX_VALUE}.
   *
   * <p>Beware that, unlike in most collections, this method is
   * <em>NOT</em> a constant-time operation. Because of the
   * asynchronous nature of these deques, determining the current
   * number of elements requires traversing them all to count them.
   * Additionally, it is possible for the size to change during
   * execution of this method, in which case the returned result
   * will be inaccurate. Thus, this method is typically not very
   * useful in concurrent applications.
   *
   * @return the number of elements in this deque
   */
  public int size() {
    int count = 0;
    for (Node<E> p = first(); p != null; p = succ(p)) {
      if (p.item != null)
      // Collection.size() spec says to max out
      {
        if (++count == Integer.MAX_VALUE) {
          break;
        }
      }
    }
    return count;
  }

  /**
   * Removes the first element {@code e} such that
   * {@code o.equals(e)}, if such an element exists in this deque.
   * If the deque does not contain the element, it is unchanged.
   *
   * @param o element to be removed from this deque, if present
   * @return {@code true} if the deque contained the specified element
   * @throws NullPointerException if the specified element is null
   */
  public boolean remove(Object o) {
    return removeFirstOccurrence(o);
  }

  /**
   * Appends all of the elements in the specified collection to the end of
   * this deque, in the order that they are returned by the specified
   * collection's iterator.  Attempts to {@code addAll} of a deque to
   * itself result in {@code IllegalArgumentException}.
   *
   * @param c the elements to be inserted into this deque
   * @return {@code true} if this deque changed as a result of the call
   * @throws NullPointerException if the specified collection or any of its elements are null
   * @throws IllegalArgumentException if the collection is this deque
   */
  public boolean addAll(Collection<? extends E> c) {
    if (c == this)
    // As historically specified in AbstractQueue#addAll
    {
      throw new IllegalArgumentException();
    }

    // Copy c into a private chain of Nodes
    Node<E> beginningOfTheEnd = null, last = null;
    for (E e : c) {
      checkNotNull(e);
      Node<E> newNode = new Node<E>(e);
      if (beginningOfTheEnd == null) {
        beginningOfTheEnd = last = newNode;
      } else {
        last.lazySetNext(newNode);
        newNode.lazySetPrev(last);
        last = newNode;
      }
    }
    if (beginningOfTheEnd == null) {
      return false;
    }

    // Atomically append the chain at the tail of this collection
    restartFromTail:
    for (; ; ) {
      for (Node<E> t = tail, p = t, q; ; ) {
        if ((q = p.next) != null &&
            (q = (p = q).next) != null)
        // Check for tail updates every other hop.
        // If p == q, we are sure to follow tail instead.
        {
          p = (t != (t = tail)) ? t : q;
        } else if (p.prev == p) // NEXT_TERMINATOR
        {
          continue restartFromTail;
        } else {
          // p is last node
          beginningOfTheEnd.lazySetPrev(p); // CAS piggyback
          if (p.casNext(null, beginningOfTheEnd)) {
            // Successful CAS is the linearization point
            // for all elements to be added to this deque.
            if (!casTail(t, last)) {
              // Try a little harder to update tail,
              // since we may be adding many elements.
              t = tail;
              if (last.next == null) {
                casTail(t, last);
              }
            }
            return true;
          }
          // Lost CAS race to another thread; re-read next
        }
      }
    }
  }

  /**
   * Removes all of the elements from this deque.
   */
  public void clear() {
    while (pollFirst() != null) {
      ;
    }
  }

  /**
   * Returns an array containing all of the elements in this deque, in
   * proper sequence (from first to last element).
   *
   * <p>The returned array will be "safe" in that no references to it are
   * maintained by this deque.  (In other words, this method must allocate
   * a new array).  The caller is thus free to modify the returned array.
   *
   * <p>This method acts as bridge between array-based and collection-based
   * APIs.
   *
   * @return an array containing all of the elements in this deque
   */
  public Object[] toArray() {
    return toArrayList().toArray();
  }

  /**
   * Returns an array containing all of the elements in this deque,
   * in proper sequence (from first to last element); the runtime
   * type of the returned array is that of the specified array.  If
   * the deque fits in the specified array, it is returned therein.
   * Otherwise, a new array is allocated with the runtime type of
   * the specified array and the size of this deque.
   *
   * <p>If this deque fits in the specified array with room to spare
   * (i.e., the array has more elements than this deque), the element in
   * the array immediately following the end of the deque is set to
   * {@code null}.
   *
   * <p>Like the {@link #toArray()} method, this method acts as
   * bridge between array-based and collection-based APIs.  Further,
   * this method allows precise control over the runtime type of the
   * output array, and may, under certain circumstances, be used to
   * save allocation costs.
   *
   * <p>Suppose {@code x} is a deque known to contain only strings.
   * The following code can be used to dump the deque into a newly
   * allocated array of {@code String}:
   *
   * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
   *
   * Note that {@code toArray(new Object[0])} is identical in function to
   * {@code toArray()}.
   *
   * @param a the array into which the elements of the deque are to be stored, if it is big enough;
   * otherwise, a new array of the same runtime type is allocated for this purpose
   * @return an array containing all of the elements in this deque
   * @throws ArrayStoreException if the runtime type of the specified array is not a supertype of
   * the runtime type of every element in this deque
   * @throws NullPointerException if the specified array is null
   */
  public <T> T[] toArray(T[] a) {
    return toArrayList().toArray(a);
  }

  /**
   * Returns an iterator over the elements in this deque in proper sequence.
   * The elements will be returned in order from first (head) to last (tail).
   *
   * <p>The returned iterator is
   * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
   *
   * @return an iterator over the elements in this deque in proper sequence
   */
  public Iterator<E> iterator() {
    return new Itr();
  }

  /**
   * Returns an iterator over the elements in this deque in reverse
   * sequential order.  The elements will be returned in order from
   * last (tail) to first (head).
   *
   * <p>The returned iterator is
   * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
   *
   * @return an iterator over the elements in this deque in reverse order
   */
  public Iterator<E> descendingIterator() {
    return new DescendingItr();
  }

  private abstract class AbstractItr implements Iterator<E> {

    /**
     * Next node to return item for.
     */
    private Node<E> nextNode;

    /**
     * nextItem holds on to item fields because once we claim
     * that an element exists in hasNext(), we must return it in
     * the following next() call even if it was in the process of
     * being removed when hasNext() was called.
     */
    private E nextItem;

    /**
     * Node returned by most recent call to next. Needed by remove.
     * Reset to null if this element is deleted by a call to remove.
     */
    private Node<E> lastRet;

    abstract Node<E> startNode();

    abstract Node<E> nextNode(Node<E> p);

    AbstractItr() {
      advance();
    }

    /**
     * Sets nextNode and nextItem to next valid node, or to null
     * if no such.
     */
    private void advance() {
      lastRet = nextNode;

      Node<E> p = (nextNode == null) ? startNode() : nextNode(nextNode);
      for (; ; p = nextNode(p)) {
        if (p == null) {
          // p might be active end or TERMINATOR node; both are OK
          nextNode = null;
          nextItem = null;
          break;
        }
        E item = p.item;
        if (item != null) {
          nextNode = p;
          nextItem = item;
          break;
        }
      }
    }

    public boolean hasNext() {
      return nextItem != null;
    }

    public E next() {
      E item = nextItem;
      if (item == null) {
        throw new NoSuchElementException();
      }
      advance();
      return item;
    }

    public void remove() {
      Node<E> l = lastRet;
      if (l == null) {
        throw new IllegalStateException();
      }
      l.item = null;
      unlink(l);
      lastRet = null;
    }
  }

  /**
   * Forward iterator
   */
  private class Itr extends AbstractItr {

    Node<E> startNode() {
      return first();
    }

    Node<E> nextNode(Node<E> p) {
      return succ(p);
    }
  }

  /**
   * Descending iterator
   */
  private class DescendingItr extends AbstractItr {

    Node<E> startNode() {
      return last();
    }

    Node<E> nextNode(Node<E> p) {
      return pred(p);
    }
  }

  /**
   * A customized variant of Spliterators.IteratorSpliterator
   */
  static final class CLDSpliterator<E> implements Spliterator<E> {

    static final int MAX_BATCH = 1 << 25;  // max batch array size;
    final ConcurrentLinkedDeque<E> queue;
    Node<E> current;    // current node; null until initialized
    int batch;          // batch size for splits
    boolean exhausted;  // true when no more nodes

    CLDSpliterator(ConcurrentLinkedDeque<E> queue) {
      this.queue = queue;
    }

    public Spliterator<E> trySplit() {
      Node<E> p;
      final ConcurrentLinkedDeque<E> q = this.queue;
      int b = batch;
      int n = (b <= 0) ? 1 : (b >= MAX_BATCH) ? MAX_BATCH : b + 1;
      if (!exhausted &&
          ((p = current) != null || (p = q.first()) != null)) {
        if (p.item == null && p == (p = p.next)) {
          current = p = q.first();
        }
        if (p != null && p.next != null) {
          Object[] a = new Object[n];
          int i = 0;
          do {
            if ((a[i] = p.item) != null) {
              ++i;
            }
            if (p == (p = p.next)) {
              p = q.first();
            }
          } while (p != null && i < n);
          if ((current = p) == null) {
            exhausted = true;
          }
          if (i > 0) {
            batch = i;
            return Spliterators.spliterator
                (a, 0, i, Spliterator.ORDERED | Spliterator.NONNULL |
                    Spliterator.CONCURRENT);
          }
        }
      }
      return null;
    }

    public void forEachRemaining(Consumer<? super E> action) {
      Node<E> p;
      if (action == null) {
        throw new NullPointerException();
      }
      final ConcurrentLinkedDeque<E> q = this.queue;
      if (!exhausted &&
          ((p = current) != null || (p = q.first()) != null)) {
        exhausted = true;
        do {
          E e = p.item;
          if (p == (p = p.next)) {
            p = q.first();
          }
          if (e != null) {
            action.accept(e);
          }
        } while (p != null);
      }
    }

    public boolean tryAdvance(Consumer<? super E> action) {
      Node<E> p;
      if (action == null) {
        throw new NullPointerException();
      }
      final ConcurrentLinkedDeque<E> q = this.queue;
      if (!exhausted &&
          ((p = current) != null || (p = q.first()) != null)) {
        E e;
        do {
          e = p.item;
          if (p == (p = p.next)) {
            p = q.first();
          }
        } while (e == null && p != null);
        if ((current = p) == null) {
          exhausted = true;
        }
        if (e != null) {
          action.accept(e);
          return true;
        }
      }
      return false;
    }

    public long estimateSize() {
      return Long.MAX_VALUE;
    }

    public int characteristics() {
      return Spliterator.ORDERED | Spliterator.NONNULL |
          Spliterator.CONCURRENT;
    }
  }

  /**
   * Returns a {@link Spliterator} over the elements in this deque.
   *
   * <p>The returned spliterator is
   * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
   *
   * <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT},
   * {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}.
   *
   * @return a {@code Spliterator} over the elements in this deque
   * @implNote The {@code Spliterator} implements {@code trySplit} to permit limited parallelism.
   * @since 1.8
   */
  public Spliterator<E> spliterator() {
    return new CLDSpliterator<E>(this);
  }

  /**
   * Saves this deque to a stream (that is, serializes it).
   *
   * @param s the stream
   * @throws java.io.IOException if an I/O error occurs
   * @serialData All of the elements (each an {@code E}) in the proper order, followed by a null
   */
  private void writeObject(java.io.ObjectOutputStream s)
      throws java.io.IOException {

    // Write out any hidden stuff
    s.defaultWriteObject();

    // Write out all elements in the proper order.
    for (Node<E> p = first(); p != null; p = succ(p)) {
      E item = p.item;
      if (item != null) {
        s.writeObject(item);
      }
    }

    // Use trailing null as sentinel
    s.writeObject(null);
  }

  /**
   * Reconstitutes this deque from a stream (that is, deserializes it).
   *
   * @param s the stream
   * @throws ClassNotFoundException if the class of a serialized object could not be found
   * @throws java.io.IOException if an I/O error occurs
   */
  private void readObject(java.io.ObjectInputStream s)
      throws java.io.IOException, ClassNotFoundException {
    s.defaultReadObject();

    // Read in elements until trailing null sentinel found
    Node<E> h = null, t = null;
    Object item;
    while ((item = s.readObject()) != null) {
      @SuppressWarnings("unchecked")
      Node<E> newNode = new Node<E>((E) item);
      if (h == null) {
        h = t = newNode;
      } else {
        t.lazySetNext(newNode);
        newNode.lazySetPrev(t);
        t = newNode;
      }
    }
    initHeadTail(h, t);
  }

  private boolean casHead(Node<E> cmp, Node<E> val) {
    return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
  }

  private boolean casTail(Node<E> cmp, Node<E> val) {
    return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);
  }

  // Unsafe mechanics

  private static final sun.misc.Unsafe UNSAFE;
  private static final long headOffset;
  private static final long tailOffset;

  static {
    PREV_TERMINATOR = new Node<Object>();
    PREV_TERMINATOR.next = PREV_TERMINATOR;
    NEXT_TERMINATOR = new Node<Object>();
    NEXT_TERMINATOR.prev = NEXT_TERMINATOR;
    try {
      UNSAFE = sun.misc.Unsafe.getUnsafe();
      Class<?> k = ConcurrentLinkedDeque.class;
      headOffset = UNSAFE.objectFieldOffset
          (k.getDeclaredField("head"));
      tailOffset = UNSAFE.objectFieldOffset
          (k.getDeclaredField("tail"));
    } catch (Exception e) {
      throw new Error(e);
    }
  }
}
